[yocto] which yocto doc explains bitbake assignment variations?

[Rudolf Streif]

Robert,

I don't think that the YP manuals do. That's more Bitbake. However, here is
some copied out of my docs. (List excuse the formatting. It's copied from
an HTML page.)


   -

   *Variable Setting (=)*

   Set a variable by assigning it a value using the *=* sign. Values must
   be surrounded by double quotes.

   *
   VAR = "value"
           *

   In this example, *VAR* is set to *value*.
   -

   *Early Default Value Assignment (?=)*

   A variable can be early-assigned a default value using the *?=* assignment
   operator:

   *
   A ?= "1"
   B ?= "2"
   B ?= "3"
           *

   In this example *A* will contain *1* if it was not previously set. *B* will
   contain *2* because the *?=* assignment operator is immediate meaning
   that if there are multiple *?=* assignments to a single variable, the
   first one will be used.
   -

   *Late Default Value Assignment (??=)*

   A variable can be late-assigned a default value using the *??=* assignment
   operator:

   *
   A ??= "1"
   B ??= "2"
   B ??= "3"
           *

   In this example *A* will contain *1* if it was not previously set. *B* will
   contain *3* because the *??=* assignment operator is a late or lazy
   assignment operator and assignment will not take place until the end of the
   parsing process. If there are multiple *??=* assignments to a single
   variable, the last one will be used.
   -

   *Variable Expansion*

   Bitbake supports referencing a variables content similar to shell
   scripting.

   *
   A = "jumps over"
   B = "The quick brown fox ${A} the lazy dog."
   echo $B
           *

   This results in the the text *The quick brown fox jumps over the lazy
   dog.* printed to the console.
   -

   *Immediate Variable Expansion (:=)*

   Typically, Bitbake expands a variable when it is used. The *:=* assignment
   operator however expands the variable immediately when it is assigned.

   *
   A = "11"
   B = "B:${A}"
   A = "22"
   C := "C:${A}"
   echo $C $B
           *

   This results in *C:22 B:22* printed to the console since the content of *
   A* is expanded immediately on assignment to variable *C*. Variable *B* did
   not get expanded until it was actually used in the *echo*statement.
   -

   *Appending (+=) and Prepending (=+)*

   The *+=* and *=+* append and prepend variables respectively while adding
   a single space between the values.

   *
   A = "12"
   A += "34"
   B = "89"
   B =+ "67"
           *

   This results in variable *A* containing *12 34* and variable *B*
    containing *67 89*.
   -

   *Appending (.=) and Prepending (=.) without Spaces*

   The *.=* and *=.* work like the above appending and prepending operators
   however without placing an additional space between the values.

   *
   A = "12"
   A .= "34"
   B = "89"
   B =. "67"
           *

   This results in variable *A* containing *1234* and variable *B*
    containing *6789*.

Rudi
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